Classification

Distribution

Endemic to the states of Kerala and Karnataka in southern India.

Its modern distribution is highly fragmented with small populations remaining in the Valapatanam, Chaliyar, Kallar, Karyangod, Kuttiyadi, Chandragiri, Sullya, Kuppam, Iritti, Anjarakandipuzha, Bhavani, and Bharatapuzha river systems.

Historical records from the Chalakudy, Periyar, Pamba, and Achankovil river systems are now considered to represent S. chalakkudiensis.

Type locality is ‘Mundakayam, Travancore, India’, which appears to correspond to the Manimala River near the town of Mundakayam in Kottayam District, Kerala state, southern India.

Habitat

A stream and river-dwelling species most often found in pristine, highly-oxygenated headwaters and upper parts of river basins where it typically congregates in rocky pools with dense riparian vegetation. It is supposedly more active at dusk and dawn than during daylight hours.

Maximum Standard Length

Maintenance

Not difficult to keep in a well-maintained set-up, and does not tend to harm aquatic plants.

Since it naturally occurs in pristine habitats it is intolerant to accumulation of organic pollutants and requires more-or-less spotless water in order to thrive. It also does best if there is a high proportion of dissolved oxygen and moderate water movement, and weekly water changes of 30-50% tank volume should be considered routine.

Water Conditions

Diet

Wild fish on a variety of worms, insects, crustaceans, plant material, and other organic debris.

In the aquarium it is easily-fed but a balanced diet comprising regular meals of small live and frozen foods such as chironomid larvae (bloodworm), Daphnia, and Artemia alongside good quality dried products will being about optimal condition and colours. Sinking foods are best.

It is said that the red pigmentation can be intensified by feeding a diet rich in carotenoids such as astaxanthin.

Generally peaceful making it an ideal resident of the well-researched community aquarium.

It is a schooling species by nature, and at least 6-10 specimens should be purchased.

The similar-looking S. chalakkudiensis is occasionally traded as S. denisonii (see ‘Notes’) and is known to be more belligerent.

Sexual Dimorphism

Adult females tend to grow a little larger, are heavier-bodied, and a little less colourful than males.

Reproduction

Large numbers are produced for the aquarium hobby in Southeast Asia and Eastern Europe, presumably via stimulation with hormones.

A 2011 study of its reproductive biology revealed that the sex ratio in wild fish appears skewed in favour of males and that absolute fecundity, i.e., the total number of eggs per female at a given point in time, is relatively low compared with some relatives such as Systomus sarana or Rasbora daniconius. Such factors, combined with habitat degradation via pollution or alteration, are likely to have an adverse effect on natural recruitment while affecting population dynamics and potentially leading to a reduced number of individuals in a given population.

In terms of private success at least one report of ‘accidental’ reproduction exists, in which a couple of fry were discovered hiding among plants during aquarium maintenance.

A more detailed report was published in the German magazine Aqualog in 2005. In this case a group of 15 adults spawned in soft, acidic water (gH 2-3/pH 5.7), depositing their eggs in a clump of Java moss (Taxiphylum barbieri). Apparently several of the adults exhibited a colour change with the dorsal surface becoming bluish, and the event appeared to be triggered by a gradual lowering of the pH via addition of driftwood.

Chester Zoo Aquarium in England have also reported successful breeding, and their theory is that a large group is needed as spawning is hypothesised to occur en masse.

This species has become an extremely popular aquarium fish since it first appeared in the ornamental trade during the late 1990s, and it has been sold under various names including ‘Denison’s barb’, ‘denisoni barb’, ‘Denison’s flying fox’, ‘rose line shark’, ‘bleeding-eye barb’, ‘red flash barb’, and ‘Indian flasher barb’. In India it is known locally as ‘Miss Kerala’ and ‘Chorai Kanni’ ( literally ‘bleeding eyes’).

It has been artificially hybridised with a member of thegenus Dawkinsia (R. Collins, pers. comm.) and a selectively-bred ‘gold’ colour form has also been produced.

During the 2000s it was the Indian state of Kerala’s most important export but collection of wild fish is now prohibited to an extent. Both S. denisoniiandS. chalakkudiensis are listed as ‘Endangered’ in the IUCN Red List ofThreatened Species, and wild stocks may have dwindled by as much as 50% in the last 15 years or so. Collection for the aquarium trade has been held largely responsible, although habitats are also being degraded by pollution from agricultural and domestic sources, plus destructive fishing methods involving explosives or organic toxins.

In 2011 the ‘closed season’ for collection was June, July, and October, based on the assumption that spawning takes place during these months, but Solomon et al. (2011) demonstrated that breeding occurs from October to March and collection should instead be prohibited between these months in order to adequately protect populations.

S. denisonii was described as a member of the genus Labeo and has also been placed in Barbus,Crossocheilus and most recently Puntius. The latter was formerly a polyphyletic catch-all which contained over 100 species, although this situation has been largely resolved since the turn of the century.

S. denisonii can be told apart from S. chalakkudiensis by possession of a subterminal mouth (vs. inferior), absence of a black marking in the dorsal-fin (vs. presence), and the scarlet body stripe being brighter and terminating beneath the centre of the dorsal-fin (vs. duller and terminates beneath or anterior to dorsal-fin origin).

References

Day, F., 1865 - Proceedings of the Zoological Society of London 1865(1): 286-318On the fishes of Cochin, on the Malabar Coast of India. Part II. Anacanthini.